As a disposal method for a molded article, a biodegradability disposal technique using microbe has been developed and is in the spotlight. Especially, in the biodegradability disposal method above, a method to utilize natural high polymers such as starch and protein has drawn attention in terms of its practicality. This is because the various biodegradable plastics have a problem that despite having fine quality almost comparable to conventional plastics (non-degradable or degradable-retardant), practically they cannot be decomposed quickly enough.
For instance, when a molded article made of the biodegradable plastic is thick, it takes a very long time until the molded article is completely decomposed, so practically it is not possible to produce the molded article having enough volume. Also, when the molded article made of the biodegradable plastic is used particularly as a disposable food container, composting the molded article together with food residues is the least harmful disposal method for the environment. However, actually it is difficult to compost them together since the biodegradable plastic above is decomposed much slower than the food residues. Furthermore, it is also difficult to crush the molded article to hasten the decomposition of the biodegradable plastic, because normally the molded article cannot be crushed easily when it has a certain thickness and strength. Thus it is almost impossible to compost the molded article made of the biodegradable plastic.
Whereas starch and protein, etc. are positively evaluated as the materials because of advantages such as:                with fine biodegradability, decomposition is quite easy even if the volume is large;        the resources can be acquired easily on account of availability of vegetable starch that is mass-produced by agriculture; and        a molded article with adequate thickness and thermal insulation can be acquired, since the molded article is usually used as an expanded molded article.        
(1) Japanese Laid-Open Patent Application. No. 5-320401/1993 (Tokukaihei 5-320401; published on Dec. 3, 1993, (2) Japanese Laid-Open Patent Application No. 7-224173/1995 (Tokukaihei 7-224173; published on Aug. 22, 1995), (3) Japanese Laid-Open Patent Application No. 7-10148/1995 (Tokukaihei 7-10148; published on Jan. 13, 1995), (4) Japanese Laid-Open Patent Application No. 2000-142783 (Tokukai 2000-142783; published on May 23, 2000), and (5) Japanese Laid-Open Patent Application No. 7-97545/1995 (Tokukaihei 7-97545; published on Apr. 11, 1995) disclose biodegradable disposal technologies using starch, protein, etc.
First, a molded article derived from the technology (1) or (2) has the advantages that it has better decomposability than a molded article mainly made of the biodegradable plastic and is also superior to those derived from paper/pulps in its diversity of the molded shape, since natural starch is used as the main ingredient. However, at the same time the molded article derived from the technology (1) or (2) has the disadvantages that it can be used only for limited purposes and is required to barrier moisture during storage, due to its poor water and moisture resistance.
Second, a molded article derived from the technology (3) or (4) is mainly made of starch or similar polysaccharide, and to enhance its water resistance, a natural resin (dammar resin, shellac resin, etc.) is coated on the surface of the molded article to form a water-resistant coating film.
However, the surface of the molded article (including an expanded molded article) mainly made of starch cannot be completely smoothed, and generation of small irregularities cannot be avoided. Thus small pinholes are likely to be formed on the surface corresponding to the irregularities on the water-resistant coating film if it is simply coated. It could be possible to render the molded article water repellent but difficult to make the same completely water resistant. Particularly, if the molded article is required to be moisture-resistant, moisture is likely to be absorbed from the pinholes on the water-resistant coating film, and the molded article tends to become disfigured.
Furthermore, the dammar resin or the shellac resin must be dissolved in an organic solvent such as alcohol, etc., when applied to the surface. This causes problems in terms of a manufacturing facility. For instance, when the organic solvent is removed after the coating, large-scale equipment is required to prevent diffusion of the organic solvent in the air that causes air and environment pollution.
Now, on a surface of a molded article derived from the technology (5) that is made of poorly water-resistant biodegradable material such as starch, as in the cases of the molded articles derived from the technologies (3) and (4), a biodegradable coating agent composed of aliphatic polyester being dissolved in halogenated hydrocarbon is coated. In this case, using a dip method (dip coating method) for actual coating of the surface, an adequately water-resistant coating film can be formed even on a complicatedly-shaped molded article.
However, in this method, it is required to remove the halogenated hydrocarbon used to dissolve the coating agent, and as in the cases of the technologies (3) and (4), problems such as a requirement of equipment to prevent diffusion of halogenated hydrocarbon. Many halogenated hydrocarbons are often harmful for a human body and the environment, and the halogenated hydrocarbon that is specifically mentioned in the technology (5) contains CFC so that it should be released to the air as little as possible. On this account, a large-scale hermetic room and a reclaiming device are required as the equipment above.
In addition to the technologies introduced above, there is a technology in which wax or hydrophobic protein, prepared as a coating solution, is coated on the surface of the molded article. Generally speaking, it is difficult to coat a water-resistant coating film on the surface of the molded article evenly and entirely, while coating on a flat molded article such as a flat plate is relatively easy. However, small irregularities are likely to be formed on the surface of the molded article mainly made of starch as described above and obstruct the formation of an even film, and furthermore, the molded article or a coating device has to be rotated when the molded article is substantially circular in cross section, for instance, like a cup or a bowl. Therefore the coating becomes more difficult.
Besides, even if the coating agent can be applied evenly and entirely by using the dip method, the coating agent runs down before it solidifies and becomes a coating film, and unevenness is likely to show up on the coating film.
The wax has a problem of poor heat resistance due to its relatively low melting point. In the meantime, although the hydrophobic protein has better heat resistance and does not need the organic solvent, the molded article absorbs water and is softened/deformed in the coating process due to frequent uses of aqueous solvents.
So, a technology that has been proposed to laminate a water-resistant coating film instead of coating thereof, more specifically, (6) Japanese Laid-Open Patent Application No. 11-171238/1999 (Tokukaihei 11-171238; published on Jun. 29, 1999), (7) Japanese Laid-Open Patent Application No. 5-278738/1993 (Tokukaihei 5-278738; published on Oct. 26, 1993), (8) Japanese Laid-Open Patent Application No. 5-294332/1993 (Tokukaihei 5-294322; published on Nov. 9, 1993).
A container of the technology (6), made by a pulp molding method instead of molding starch, is coated by a water-impermeable or non-absorbing protective layer. This method has the advantage that the conventional plastic coating method can be applied almost without any change. However, at the same time the method has problems such as:                the pulp molding slowly biodegrades since it is mainly made of fiber so that the molded article cannot be disposed together with remaining foods, etc.; and        only limited types of molded articles can be produced because it is difficult to make the molded article thicker, and also the molded article is not suitable for a deep drawing.        
Meanwhile, to make a biodegradable container, a thin film made of biodegradable plastic is coated on a surface of the biodegradable container of the technologies (7) or (8) made of either one of natural polysaccharide or protein, or either of the two materials that is chemically modified but still biodegradable.
In this technology, while the biodegradable plastic is provided as the thin water-resistant coating film, the container itself is made of natural polysaccharide or protein, etc. with enough thickness. On this account, the container is sufficiently water-resistant as well as biodegradable. Thus this technology is particularly promising among the disposal technologies by biodegradability using starch or protein, etc.
However, the technology (7) is an arrangement that the biodegradable plastic thin film simply coats the main body of the biodegradable container, and a concrete arrangement of the biodegradable container is hardly mentioned.
For instance, when the main body of the biodegradable container is mainly made of polysaccharide or protein, strength of the main body should be cared of, but the technology (7) does not explain about the strength at all, and also does not explain how the biodegradable plastic thin film is actually coated, for instance, by forming it by the coating method or by attaching preformed film, etc.
Moreover, the technology (7) does not stipulate the coating state of the biodegradable plastic thin film on the main body of the biodegradable container at all. The biodegradable plastic thin film coats the main body of the biodegradable container mainly made of polysaccharide or protein, to improve water resistance of the main body. But the technology (7) does not mention how the main body is coated, except that it is simply coated.
Even if the biodegradable container is used as a disposable one, the container should still have a stability and durability as a one-way container. So the biodegradable plastic thin film should not fall off from the main body of the biodegradable container. The coating state on the main body is an important factor, but no description with respect to this can be found in (7).
Furthermore, as already described, it is difficult to use biodegradable plastics for a thick molded article due to its slow biodegradability. The speed of the biodegradability also greatly depends on not only the thickness of the molded article but also a total amount of biodegradable plastics contained in the molded article. The technology (7) only describes that effectiveness of the biodegradability is improved if the main body of the biodegradable container is expanded, and there are no comments on a relationship between a degree of the expansion and the biodegradability, and a balance between the biodegradability of the biodegradable plastic and that of the main body of the biodegradable container. As a result, it is not possible to manage the biodegradability of the whole container favorably.
In the meantime, the technology (8) can be assumed to correspond to one of the manufacturing technologies of the biodegradable container disclosed in the technology (7). In this technology, a thermoplastic is dissolved in a solvent and coated on the surface of the main body of the biodegradable container. Then after the solvent is dried and volatilized, another coating thin film made of thermoplastic is laminated and bonded by thermo compression. That is to say, the technology (8) discloses that thermoplastic is used as an adhesive to bond the coating thin film (equivalent to the biodegradable plastic thin film) securely.
Now, as described in relation to technologies (3) to (5), when the thermoplastic dissolved in the solvent is used, problems than an equipment to prevent diffusion of the solvent is required. Moreover, an embodiment of the technology (8) uses chloroform as the solvent, which should be scattered in the air as little as possible. Thus, as in the case of the technology (5), a large-scale hermetic room and reclaiming device are required as the equipment above.
Also, the manufacturing process of the technology (8) acquires the main body of the biodegradable container by press-molding a sheet made of polysaccharide or protein that is preformed by a metal mold. Thus it is impossible to mold molded articles such as a container with deep drawing shape like a cup, or molded articles having irregular thickness like a food tray with partitions or a wrapping tray, and molded articles having complicated shape like cushioning materials for wrapping.
Other biodegradable containers or biodegradable molded articles are publicly known as follows;
(10) Japanese Translations of PCT International Laid-Open Patent Application No. WO-97/10293 (Tokuhyohei 11-512467; published on Mar. 20, 1997), (11) Japanese Translations of PCT International Laid-Open Patent Application No. WO-94/05492 (Tokuhyohei 8-500547; published on Mar. 17, 1994), and (12) Japanese Laid-Open Patent Application No. 6-125718/1994 (Tokukaihei 6-125718; published on May 10, 1994).
In addition, a method to fill polyurethane foam using a bag, that is, (13) Japanese Laid-Open Patent Application No. 63-54217/1988 (Tokukaisho 63-54217; published on Mar. 8, 1988), has been publicly known.
Also, it has been publicly known that depressure is carried out at vacuum molding of typical foamed plastic;
(14) Japanese Laid-Open Patent Application No. 52-134670/1977 (Tokukaisho 52-134670; published on Nov. 11, 1977), (15) Japanese Laid-Open Patent Application No. 54-127476/1979 (Tokukaisho 54-127476; published on Oct. 3, 1979), (16) Japanese Laid-Open Patent Application No. 55-73535/1980 (Tokukaisho 55-73535; published on Jun. 3, 1980), and (17) Japanese Laid-Open Patent Application No. 57-1712/1982 (Tokukaisho 57-1712; published on Jan. 6, 1982).
The present inventors have applied the invention relating to a method for manufacturing a biodegradable molded article including a simultaneous attaching process that a coating film is finally attached to a surface of the biodegradable molded article by using a slurry or dough molding material mainly made of starch or a derivative thereof and water added thereto and a coating film mainly made of biodegradable plastic and having at least hydrophobicity, heating the molding material and the coating film in the mold and molding the biodegradable expanded molded article into a specified shape through steam expansion, and at the same time heating, softening and press bonding the coating film (Refer to (9) Application No. WO 02/22353 A1 published on Mar. 21, 2002, PCT International Laid-Open Patent Application No. PCT/JP01/07903 dated Sep. 12, 2001, when was not published on the application date that is a basis of the priority claim of the present invention, and thereafter it was published.)
The above prior invention is an excellent one that can manufacture a biodegradable molded article having excellent biodegradability and water resistance and reduce manufacturing processes and time.
In the method of Patent (9), the molding material placed between the coating films is expanded inside a mold through steam expansion and presses the coating film against the mold. Steam occurring from the molding material is exhausted out of the mold through a canal-formed exhausting part formed at a joint part or a fitting part between mold halves of the mold from between the coating films. However, if an enclosed space is formed between the coating film and the mold, air is pooled and cannot be escaped. Therefore, pressing of the coating film against the mold stops when pressure by expansion of the molding material and the pressure of air pooled in the enclosed space is balanced. Moreover, the coating film is not expanded all over the mold and is not fit to the surface of the mold by air pooled in the enclosed space. Thus, the biodegradable molded article may not be shaped into the cavity of the mold by the air pooled in the enclosed space. Especially, this appears at a part where at least a certain area of both concave part and smooth surface continue. In the result, in the biodegradable molded article to be molded, desirable thickness was not obtained at the concave part of the mold or slight asperity appeared on a surface of the part where at least a certain area of smooth surface continues. Thus, the biodegradable molded article has insufficient strength and the biodegradable molded article having fine appearance and printability was not obtained.
In addition, in the invention of the reference (9), in case that the biodegradable molded article having a deep drawing shape such as a bowl-shaped container or a cup-shaped container is manufactured, the coating film is preformed in the substantially same shape as the outer shape of the biodegradable molded article or the coating film was divided into film pieces as the development elevation so that the it may be formed in the substantially same shape as the outer shape of the biodegradable molded article. Therefore, at least two processes were necessary to make the biodegradable molded article with deep drawing shape from the coating film.